Internal combustion engines utilize feedback from Exhaust Gas Oxygen (EGO) sensors to maintain desired air-fuel ratio mixtures during combustion, at least under some conditions. Various types of EGO sensors may be used, such as linear type sensors, sometimes referred to as Universal Exhaust Gas Oxygen (UEGO) sensors, and switching type sensors such as Heated Exhaust Gas Oxygen (HEGO) and Exhaust Gas Oxygen (EGO) sensors, depending on whether a heater is included.
In internal combustion engines equipped with an exhaust catalyst to reduce undesirable emissions, it has been found that modulation of the air-fuel ratio to rich and lean of stoichiometric conditions may improve efficiency of the catalyst under some conditions. One application of EGO sensors is to provide feedback upon which air-fuel ratios may be modulated. One prior approach involved modulating the air-fuel ratio using feedback from a Catalyst Monitor Sensor (CMS) such as a HEGO sensor to identify the stoichiometric conditions around which modulation was to take place. However, during warm up of the catalyst, the modulation controller is unable to use the HEGO CMS for feedback. As a result, during warm up the catalyst may operate at a less than desired efficiency.
It is also known that while an internal combustion engine warms up, both the constituent emissions from the engine as well as the catalyst efficiency toward these various constituent emissions change over time. One problem that occurs is that the catalyst can sometimes become saturated with a particular constituent during warm-up and early stable engine operation. As a result, the engine and catalyst system may operate at a less than desired efficiency during warm up and early stable engine operation.